This invention relates to a wear-resistant coating applied to machine component surfaces which are exposed to frictional wear such as, for example, the running faces of piston rings for internal combustion engines. Such coating is usually applied by a flame spraying process, preferably a plasma spraying process, with open spraying or in-chamber spraying. The wear-resistant coatings, whose primary purpose is to extend the service life of machine components which are exposed to extreme wear, are formed preferably of metal, metal/ceramic mixtures and/or pure ceramic materials. On the running faces of piston rings particularly molybdenum or molybdenum-containing layers applied by a flame spraying (plasma spraying) method have been proved advantageous. These coatings are provided either on the entire running face or in grooves in an in-chamber spraying method.
In case of extreme stresses on piston rings, occurring, for example, during a dry run in a damaged engine or in up-to-date high-rpm diesel engines, particularly turbocharged engines, ruptures may occur on the molybdenum layers as a result of overheating which may lead to a scaling or a break-off of the coating. For this reason, particularly molybdenum alloys or other alloys and ceramic materials are often used with an additive of low melting point alloys or intermetallic compounds as a binder material. Such measures, however, have achieved only partially the desired results.
Coatings on faces of machine components exposed to frictional wear must have the following properties in addition to a satisfactory and temperature-resistant adhesion to the substrate and a satisfactory and temperature-resistant cohesion of the coating material itself: the outer surface of the coating must be scorch mark-resistant and wear resistant and in such areas it should have pores for receiving lubricants. Further, in case of a dry (non-lubricated) run, the coating should exhibit a sufficient self lubricating property and also, during the run-in period its own wear should be sufficient to adapt to the frictional counter faces. Furthermore, such layers also should have a high break-off resistance and even after a long service they should exhibit only slight fatigue, if any at all. Similarly, particularly in case of in-chamber sprayed coatings, the thermal coefficient of expansion of the substrate and that of the coating material should be coordinated to prevent the generation of stresses which may also result in the breaking away of the coating. The alloy or ceramic layers developed heretofore, however, do not, in most cases, have simultaneously all the above-discussed properties. Measures such as the addition of a hard metal may improve the wear resistance of the layer, but it has led to a decrease in the adhesion of the layer to the substrate or the cohesion within the coating so that such layers did not prove to be resistant to break-off or to be insensitive to thermal shocks.
In the coating technology involving piston rings, such as applying a hard chromium layer galvanically, it is conventional to provide intermediate layers at the substrate for improving the hard chromium layer or to provide the upper surfaces with soft metal layers for improving the run-in behavior of the rings. While these measures are effective in many cases, the application of intermediate layers or the run-in surface layers requires additional process steps which render the rings more expensive. Further, because of the unsatisfactory adhesion of the individual layers to one another, damages are likely to occur during heavy duty service.